I'm not sure what you are referring to about what would work. Bear in mind that most people when installing a major upgrade like from SL to ML or higher, would take the opportunity to do a clean install and this is probably a good thing to do.

However if you wanted to upgrade SL to Mavericks and if it were possible to let the upgrade package do the whole thing without doing a clean install, then you would certainly want to do that on the sandbox and see if any funny business ensued.

But if you are nervous about upgrading then you could always simply buy another 500GB external drive which are pretty cheap to keep a clone of your SL set up until you felt whatever it was that you were reluctant about was not going to happen. Or maybe you have software that you really want to keep using and is not longer supported on a higher OS, well in that case you could keep the aforementioned extra SL clone forever just to use with that software.

I'm not really sure what all the "SL is the last decent iteration of OS X" is all about really. But then I went from Leopard to Lion then to ML then to Mavericks and skipped SL, but when I see all the whinging about how fantastically marvellous 'Spaces' was in SL and how it is completely banjaxed on ML, I wonder about the sanity of the world.